Conversion of olefins to gasoline and/or distillate product is disclosed in U.S. Pat. Nos. 3,960,978 and 4,021,502 (Givens,Plank and Rosinski) wherein gaseous olefins in the range of ethylene to pentene, either alone or in admixture with paraffins, are converted into an olefinic gasoline blending stock by contacting the olefins with a catalyst bed made up of ZSM-5 or related zeolite. In U.S. Pat. Nos. 4,150,062 and 4,227,992 Garwood et al discloses the operating conditions for the Mobil Olefin to Gasoline/Distillate (MOGD) process for selective conversion of C.sub.3 + olefins. A fluidized bed process for converting ethene-containing light olefinic streams, sometimes referred to as the Mobil Olefin to Gasoline (MOG) process is described by Avidan et al in U.S. Patent Application 006,407 filed 23 Jan 1987, now U.S. Pat. No. 4,746,762. The phenomena of shape-selective polymerization are discussed by Garwood in ACS Symposium Series No. 218, Intrazeolite Chemistry, "Conversion of C.sub.2 -C.sub.10 to Higher Olefins over Synthetic Zeolite ZSM-5", 1983 American Chemical Society.
In the process for catalytic conversion of olefins to heavier hydrocarbons by catalytic oligomerization using an acid crystalline metallosilicate zeolite, such as ZSM-5 or related shape selective catalyst, process conditions can be varied to favor the formation of either gasoline or distillate range products. In the gasoline operating mode, or MOG reactor system, ethylene and the other lower olefins are catalytically oligomerized at elevated temperature and moderate pressure. Under these conditions ethylene conversion rate is greatly increased and lower olefin oligomerization is nearly complete to produce C.sub.5 + hydrocarbons in good yield.
The olefins contained in an FCC gas plant are an advantageous feed for MOG. U.S. Pat. No. 4,090,949 discloses upgrading olefinic gasoline by conversion in the presence of carbon hydrogen-contributing fragments including olefins and a zeolite catalyst and where the contributing olefins may be obtained from a gas plant. U.S. Pat. Nos. 4,471,147 and 4,504,691 disclose an MOG/D process using an olefinic feedstock derived from FCC effluent. In these two latter patents the first step involves prefractionating the olefinic feedstock to obtain a gaseous stream rich in ethylene and a liquid stream containing C.sub.3 + olefin.
The conventional MOG process design is concerned with converting ethylene in a fuel gas stream, such as an FCC off-gas, to gasoline. In the conventional MOG design no LPG recovery facility is provided since the LPG content of the MOG reactor effluent is relatively small. However, when it is desired to convert propene and/or butene to gasoline by processing olefinic-paraffinic LPG the unreacted paraffinic LPG, unconverted olefinic LPG and LPG produced in the conversion step constitute a significant portion of the MOG reactor effluent. In this case, processing the reactor effluent in the conventional MOG design is unacceptable since a major portion of reactor effluent LPG will be lost to fuel gas. However, with an adequate recovery and separation design for the LPG content of an MOG process converting C.sub.2-C.sub.4 olefins the performance of the MOG process could be improved where the process would represent a viable alternative to acid catalyzed alkylation as a route to high octane gasoline. Further, an economical recovery and separation step will open up the MOG process to utilize a wider range of available feedstock, particularly FCC light olefinic products, routinely available in the refinery setting. The provision of an improved MOG process as an alternative to the economically and environmentally beleaguered alkylation process would constitute a very noteworthy contribution to the options available to the refinery arts for the production of high octane.
Accordingly, it is an object of the present invention to provide an improved process for the conversion of light olefins, particularly C.sub.2 -C.sub.4 olefins, to high octane gasoline.
Another object of the present invention is to present a useful design for the separation of LPG components from the reactor effluent and olefins to gasoline process.
Another object of the present invention is to provide the foregoing improved LPG recovery and separation process as an integral part of an FCC unsaturated gas plant and thereby confer improved economies upon the integrated process.